The present invention relates to writing instruments having a writing tip supplied with fluid ink, such as felt pens and the like.
More specifically, the invention relates to a felt pen or like writing instrument comprising an elongate hollow body in which there is defined a first chamber for housing a plug of hydrophilic material associated with a writing tip one end of which projects externally from the body, and a second chamber separated from the first chamber, which defines a liquid ink reservoir for supplying the writing tip.
A writing instrument of the type defined above is the subject of European Patent Application EP-516538. The writing tip of the writing instrument described in this document is long so that it passes through a significant portion of the body, having a writing end which projects externally from the body and an opposite end which extends into the second chamber, which constitutes the liquid ink reservoir. As a result of this construction of the writing tip, the liquid ink is supplied directly to the writing tip from the reservoir. A tubular plug is also associated with the writing tip, arranged in the first chamber, in an intermediate position between the writing end of the tip and the second chamber, which the tip passes through from one end to the other. The function of the plug is to absorb any excess ink from the reservoir, should the air in the reservoir heat up, causing excess pressure to build up, and to pass on this stored ink to the writing tip once normal operating conditions are restored. In practice, this tubular plug acts as a temporary ink store only when the main reservoir causes excess ink to flow to the writing tip.
However, the structure of the writing instrument described in this earlier document has several disadvantages. Firstly, since in the most common arrangement the writing tip is made up of a bundle of parallel fibres extending axially with respect to the writing tip itself, these fibres act to allow ink to flow in a predominately axial direction, whereby the arrangement of a plug which surrounds radially the writing fibre is found to hamper the exchange of ink between the fibre and the plug. In practice, the ink tends preferably to flow along the writing tip even if excess pressure has built up in the liquid-ink reservoir, with the result that under such circumstances ink tends to drip from the end of the writing tip rather than accumulating temporarily in the plug. In addition, the manufacture of a rectilinear writing tip which is sufficiently long to project from the body at one end and extend into the ink reservoir at the other, and that of a tubular plug, as well as the operation involved in axially inserting the writing tip into the plug, pose technological problems which cannot be solved both reliably and inexpensively at the same time.